TDM provides a convenient method of transporting multiple channels of digital data via a single physical link. For example, the T1 digital carrier system has been in use since the early 1960's, providing the capability for commercial and public digital communications. The T1 digital carrier system uses two frame formats; the superframe format and the extended superframe format. One difference between the two formats is the distribution of frame synchronization bits. The superframe format embeds a frame synchronization pattern in groups of 12 contiguous frames, while the extended superframe format embeds a frame synchronization pattern in groups of 24 contiguous frames. FIG. 1 shows the structure of the superframe format of the T1 digital carrier system. In general, a T1 superframe link includes 24 distinct TDM time slots, each conveying eight bits of information. A single framing bit is included with each group of 24 TDM time slots to form a frame of 193 bits. The framing bits of 12 consecutive frames form a unique pattern, which is used by the receiving equipment to establish frame synchronization. The transmission rate of a T1 link is 1.544 million bits per second (hereinafter MBPS), so the maximum available data rate for any single time slot is 64 thousand bits per second (hereinafter KBPS). With the T1 TDM architecture, 24 distinct 64 KBPS channels can be transmitted over a single T1 link. To increase bandwidth, a single channel can utilize more than one time slot. For example, if two time slots are used for a single channel, the channel capacity increases to 128 KBPS. If all 24 time slots are used for a single channel, the system achieves the maximum channel capacity of 1.536 MBPS.
For digitized voice transmission, each voice channel is typically conveyed via a single time slot. The original analog voice signal is digitized using pulse code modulation (hereinafter PCM) at a rate of 8000 samples per second. For five of every six frames, eight bit PCM samples are transmitted in a time slot. Seven bit PCM samples are transmitted every sixth frame, with the eighth bit from each time slot being used for signaling. The signaling bits form a control data stream for each time slot, which may be used, for example, to establish a connection or terminate a call.
For digital data transmission, each time slot typically carries up to seven bits of user information, with the eighth bit used for control or unused and set to a constant value. Such an arrangement allows a channel capacity of 56 KBPS per time slot. In some applications, all eight bits of each time slot are used for user information, which allows the maximum channel capacity of 64 KBPS per time slot.
A common scheme known as subrate multiplexing is used to convey multiple low rate channels within a single T1 time slot. Subrate multiplexing uses one of the seven available bits for control, and the remaining six bits for user data, at a total channel capacity of 48 KBPS. The 48 KBPS channel conveys five 9.6 KBPS channels, or ten 4.8 KBPS channels or twenty 2.4 KBPS channels. The control bit contains information which is used by the receiver to determine which of the subrates is being utilized.
A data channel is often accompanied by a low rate status channel which conveys information regarding activity status and/or fault status of the data channel. In the aforementioned subrate multiplexing scheme, no channel capacity is available within a time slot to convey such information. Consequently, one or more additional time slots must be used to transmit the status information.
There is a need for a method and apparatus for increasing the channel capacity of the prior art subrate multiplexing scheme.
It is therefore an object of the invention to provide a method and apparatus for increasing the channel capacity of the prior art subrate multiplexing scheme.
Other objects and advantages of the present invention will become apparent upon consideration of the appended drawings and description thereof.